Method for processing signals representing color pictures

ABSTRACT

A method for processing at least a part of a color picture comprises at least one pixel defined by a plurality of color values. Each color value is variable between black and a color clip value. The method comprises, determining for each color value the quantity of the color value above the common clip value, computing a linear combination of the quantities, and for at least one of the color values, outputting a processed color value based on the linear combination.

This application claims benefit, under 35 U.S.C. 119, of European PatentOffice Application no. 02291302.4 filed May 28, 2002.

FIELD OF THE INVENTION

The invention relates to a method for processing signals representingcolour pictures.

BACKGROUND OF THE INVENTION

It is current practice for cameras to use 3 CCD arrays to capture acolour picture. Each array senses the picture through a given colourfilter. The 3 colours which are commonly used for this purpose are red(R), green (G), and blue (B).

Each CCD array outputs a video signal carrying information about thelight received by each of its pixels. The 3 video signals are thengenerally digitised and put together as a digital stream.

The light received by such a camera from a given direction is thusdefined by 3 colour values, for instance between 0 and 4,095 if coded on12 bits. These data are then further processed to get a usable videostream. The further processing steps can take place either in the cameraitself or in a studio.

One of these processing steps is applying a white balance. The goal ofthe white balance is to determine the weight of the various colours toget a picture with a desired look, generally as natural as possible.Typically white balance is performed by imaging a white referencesurface which is illuminated with light of a specific colourtemperature. The gain of each colour is adjusted until an image of thewhite reference surface is deemed to be white, i.e. non-coloured andsubstantially as viewed when illuminated with the specific colourtemperature. It consists in applying a gain specific to the colour toeach colour value.

As the range within which colour values out of CCD arrays vary is commonto the 3 colours whereas the gains for white balance are generallydifferent (depending on the colour), the range of colour values afterwhite balance is specific to each colour.

Differently said, the maximum value (corresponding to saturation of theCCD array and generally called clipping level) after white balance isgenerally different for the various colours.

Due to this difference, which can be important (e.g. a gain of 14 forred, 5 for green and 8 for blue), a big part of the dynamic range (afterwhite balance) from the colours with highest clipping level remainsunused.

SUMMARY OF THE INVENTION

An advantageous arrangement makes better use of the dynamic rangeavailable in colour values from CCD imagers to improve the dynamic rangeof the camera in highlight conditions.

In an inventive method for processing at least a part of a colourpicture comprising at least one pixel defined by a plurality of colourvalues, each colour value being variable between black and a colour clipvalue. The method employs the steps of, determining the lowest of thecolour clip values as common clip value, determining for each colourvalue the quantity of the colour value above the common clip value,computing a linear combination of the quantities. For at least one ofthe colour values, outputting as processed colour value, the colourvalue plus the linear combination if the colour value is below thecommon clip value and the common clip value plus the linear combinationif the colour value is above the common clip value.

According to a possible solution, the method applies to each colourvalue.

According to another possible solution, the method applies to the greenvalue only, whereas the red value and blue value remain untouched.

According to a preferred embodiment, the linear combination is a meanvalue of the quantities.

Further features of the invention will appear in the followingdescription of exemplary embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The single FIGURE block diagram shows a processing arrangement includingvarious inventive arrangements.

DETAILED DESCRIPTION OF PREFERED EMBODIMENTS

The exemplary inventive arrangement of FIG. 1 generally applies tocolour values (R_(in), G_(in), B_(in))_(i,j) defining pixels (i, j)after a white balance. The block diagram of FIG. 1 shows a processingarrangement including various inventive arrangements. Sensor signals,Red, Green and Blue are converted to digital words having a maximumcommon output level of MAX=2^N −1 for an N-bit analog to digitalconverter with, for example, a resolution of 12 bits. The gain of eachcolour is adjusted to achieve a white balance condition in response torespective signals R_gain, G_gain and Blue_gain derived, as is known,from manual or automatic settings. The individual gain adjustment forwhite balance may be represented by,Rmax=MAX*Gain_Red,Gmax=MAX*Gain_Green,Bmax=MAX*Gain_BlueThe channel with the lowest gain automatically has the common clipvalue:CLIP=MAX*min(Gain_Red, Gain_Green, Gain_Blue),this is a special case of the more generalised formula which followswhereCLIP=min(Rmax, Gmax, Bmax)A clipping signal is formed and applied to each colour at respectivelimiting blocks and advantageously a non-coloured enhancement signal,ENH_nocolour, is formed with contributions from each colour that has avalue that exceeds the clipping value.

If values output by CCD arrays for pixel (i, j) and digitised are called(R_(CCD), G_(CCD), B_(CCD))_(i,j), the effect of the white balance foreach pixel (i, j) can be written as:(R _(in))_(i,j) =R _(gain)·(R _(CCD))_(i,j)(G _(in))_(i,j) =G _(gain)·(G _(CCD))_(i,j)(B _(in))_(i,j) =B _(gain)·(B _(CCD))_(i,j).

For instance, R_(gain)=14, G_(gain)=5 and B_(gain)=8 can be used.

Each value (R_(CCD))_(i,j), (G_(CCD))_(i,j) and (B_(CCD))_(i,j) is forinstance coded on 12 bits and can consequently take values between 0 and4095. These values can be directly output from the CCD arrays, but theycan also be retrieved from a recording of the raw data output from theCCD arrays. In this last case, a transfer curve can be applied atrecording (to get only values of 10 bits on the recording medium); theinverse transfer curve is then applied at retrieval.

The limiting levels for each colour, called hereafter colour clip value,are thus:R _(max) =R _(gain)·4095G _(max) =G _(gain)·4095B _(max) =B _(gain)·4095.

It is clear from above that R_(max), G_(max) and B_(max) are generallydifferent from one another.

The minimum of theses clip levels or colour clip values will be calledherebelow common clip value and is thus defined as:CLIP=min(R _(max) ,G _(max) ,B _(max)).

Above this common clip value CLIP, the colour information becomesunreliable.

The information above the common clip value CLIP is used as colourlessenhancement signal as described below.

In a first embodiment of the method, the processed colour values(R_(enhanced))_(i,j), (G_(enhanced))_(i,j) and (B_(enhanced))_(i,j) foreach pixel (i, j) are computed as follows from the colour values outputby the white balance step.If (R _(in))_(i,j)<CLIP then (R _(clipped))_(i,j)=(R _(in))_(i,j) else(R _(clipped))_(i,j)=CLIP.If (G _(in))_(i,j)<CLIP then (G _(clipped))_(i,j)=(G _(in))_(i,j) else(G _(clipped))_(i,j)=CLIP.If (B _(in))_(i,j)<CLIP then (B _(clipped))_(i,j)=(B _(in))_(i,j) else(B _(clipped))_(i,j)=CLIP.

The values (R_(clipped))_(i,j), (G_(clipped))_(i,j) and(B_(clipped))_(i,j) can be called the clipped RGB data.

The colourless enhancement signal for each pixel is a linearcombination:(ENH_nocolour)_(i,j)=0.5.{(R _(in i,j) −R _(clipped i,j))+(G _(in i,j)−G _(clipped i,j))+(B _(in i,j)−B_(clipped i,j))}.

For each pixel, the colourless enhancement signal (ENH_nocolour)_(i,j)is added to the clipped RGB signal to obtain the processed (andenhanced) colours values(R _(enhanced))_(i,j)=(R _(clipped))_(i,j)+(ENH_nocolour)_(i,j)(G _(enhanced))_(i,j)=(G _(clipped))_(i,j)+(ENH_nocolour)_(i,j)(B _(enhanced))_(i,j)=(B _(clipped))_(i,j)+(ENH_nocolour)_(i,j).

Clearly, the colourless enhancement signal (ENH_nocolour)_(i,j) allowsthe colours with low clipping level (green and blue in the aboveexample) to take higher values than the colour clip level, which resultsin an increase in the dynamic range.

The processed colour values can then be used (R_(enhanced))_(i,j),(G_(enhanced))_(i,j) and (B_(enhanced))_(i,j) instead of the colourvalues (R_(in), G_(in), B_(in))_(i,j) for further processing.

The second embodiment of the method described below is particularlyadapted to pictures representing blue sky with clouds. The differencebetween blue sky and white clouds is basically a modulation in the redsignal. To maintain the blue sky we only compensate for the lack ofgreen signal caused by clipping in the green channel. Using thecolourless enhancement signal proposed above, the processed colourvalues in this embodiment are:(R _(enhanced))_(i,j)=(R _(in))_(i,j)(G _(enhanced))_(i,j)=(G _(clipped))_(i,j)+(ENH_nocolour)_(i,j)(B _(enhanced))_(i,j)=(B _(in))_(i,j).

The green signal only is corrected; red and blue signals remainuntouched.

As previously, by increasing the dynamic range of the green signal afterprocessing, the dynamic range of the camera as a whole is improved.

1. A method for processing at least a part of a colour picturecomprising at least one pixel defined by a plurality of colour values,each colour value being variable between black and a colour clip value,with the following steps: determining the lowest of the colour clipvalues as common clip value, determining for each colour value thequantity of the colour value above the common clip value, computing alinear combination of said quantities, for at least one of the colourvalues, outputting as processed colour value; the colour value plus saidlinear combination if said colour value is below the common clip value;the common clip value plus said linear combination if said colour valueis above the common clip value.
 2. A method according to claim 1, withthe steps of: outputting as processed colour value for each colourvalue; the colour value plus said linear combination if said colourvalue is below the common clip value; the common clip value plus saidlinear combination if said colour value is above the common clip value.3. A method according to claim 1, wherein said colour values are a redvalue, a green value and blue value, with the steps of: outputting asprocessed green value; the green value plus said linear combination ifsaid green value is below the common clip value; the common clip valueplus said linear combination if said green value is above the commonclip value; outputting the red value as processed red value; outputtingthe blue value as processed blue value.
 4. A method according to claim1, wherein said linear combination is a mean value of said quantities.5. A method according to claim 1, wherein said colour values aregenerated in a white balance step.
 6. A method according to claim 1,with the step of: digitising signals from CCD arrays and applying awhite balance thereto to generate the colour values.
 7. A methodaccording to claim 1, wherein the colour values are retrieved from aprevious recording.
 8. A method according to claim 7, wherein an inversetransfer curve is applied to generate the colour values.